23.4.1 Writing Unit Tests for the test package

It is preferred that tests for the test package use the
unittest module and follow a few guidelines.
One is to name the test module by starting it with "test_" and end it with
the name of the module being tested.
The test methods in the test module should start with "test_" and end with
a description of what the method is testing.
This is needed so that the methods are recognized by the test driver as
test methods.
Also, no documentation string for the method should be included.
A comment (such as
"# Tests function returns only True or False") should be used to provide
documentation for test methods.
This is done because documentation strings get printed out if they exist and
thus what test is being run is not stated.

This boilerplate code allows the testing suite to be run by
test.regrtest as well as on its own as a script.

The goal for regression testing is to try to break code.
This leads to a few guidelines to be followed:

The testing suite should exercise all classes, functions, and
constants.
This includes not just the external API that is to be presented to the
outside world but also "private" code.

Whitebox testing (examining the code being tested when the tests are
being written) is preferred.
Blackbox testing (testing only the published user interface) is not
complete enough to make sure all boundary and edge cases are tested.

Make sure all possible values are tested including invalid ones.
This makes sure that not only all valid values are acceptable but also
that improper values are handled correctly.

Exhaust as many code paths as possible.
Test where branching occurs and thus tailor input to make sure as many
different paths through the code are taken.

Add an explicit test for any bugs discovered for the tested code.
This will make sure that the error does not crop up again if the code is
changed in the future.

Make sure to clean up after your tests (such as close and remove all
temporary files).

If a test is dependent on a specific condition of the operating system
then verify the condition already exists before attempting the test.

Import as few modules as possible and do it as soon as possible.
This minimizes external dependencies of tests and also minimizes possible
anomalous behavior from side-effects of importing a module.

Try to maximize code reuse.
On occasion, tests will vary by something as small as what type
of input is used.
Minimize code duplication by subclassing a basic test class with a class
that specifies the input: